Tuesday, November 08, 2016

Dazzling eyelid-like features bursting with stars in galaxy IC 2163 formed from a tsunami of stars and gas triggered by a glancing collision with galaxy NGC 2207 (a portion of its spiral arm is shown on right side of image). ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of the galaxy. Credit: M. Kaufman; B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble Space Telescope

Annotated image showing dazzling eyelid-like features bursting with stars in galaxy IC 2163 formed from a tsunami of stars and gas triggered by a glancing collision with galaxy NGC 2207 (a portion of its spiral arm is shown on right side of image). ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of the galaxy. Credit: M. Kaufman; B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble Space Telescope

Galaxies IC 2163 (left) and NGC 2207 (right) recently grazed past each other, triggering a tsunami of stars and gas in IC 2163 and producing the dazzling eyelid-like features there. ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of the galaxy pair. Credit: M. Kaufman; B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble Space Telescope

Astronomers using the Atacama Large Millimeter/submillimeter Array
(ALMA) have discovered a tsunami of stars and gas that is crashing
midway through the disk of a spiral galaxy known as IC 2163. This
colossal wave of material – which was triggered when IC 2163 recently
sideswiped another spiral galaxy dubbed NGC 2207 – produced dazzling
arcs of intense star formation that resemble a pair of eyelids.

“Although
galaxy collisions of this type are not uncommon, only a few galaxies
with eye-like, or ocular, structures are known to exist,” said Michele
Kaufman, an astronomer formerly with The Ohio State University in
Columbus and lead author on a paper published in the Astrophysical Journal.

Kaufman
and her colleagues note that the paucity of similar features in the
observable universe is likely due to their ephemeral nature. “Galactic
eyelids last only a few tens of millions of years, which is incredibly
brief in the lifespan of a galaxy. Finding one in such a newly formed
state gives us an exceptional opportunity to study what happens when one
galaxy grazes another,” said Kaufman.

The interacting pair of
galaxies resides approximately 114 million light-years from Earth in the
direction of the constellation Canis Major. These galaxies brushed past
each other – scraping the edges of their outer spiral arms – in what is
likely the first encounter of an eventual merger.

Using ALMA’s
remarkable sensitivity and resolution, the astronomers made the most
detailed measurements ever of the motion of carbon monoxide gas in the
galaxy’s narrow eyelid features. Carbon monoxide is a tracer of
molecular gas, which is the fuel for star formation.

The data
reveal that the gas in the outer portion of IC 2163’s eyelids is racing
inward at speeds in excess of 100 kilometers a second. This gas,
however, quickly decelerates and its motion becomes more chaotic,
eventually changing trajectory and aligning itself with the rotation of
the galaxy rather than continuing its pell-mell rush toward the center.

“What
we observe in this galaxy is very much like a massive ocean wave
barreling toward shore until it interacts with the shallows, causing it
to lose momentum and dump all of its water and sand on the beach,” said
Bruce Elmegreen, a scientist with IBM’s T.J. Watson Research Center in
Yorktown Heights, New York, and co-author on the paper.

“Not
only do we find a rapid deceleration of the gas as it moves from the
outer to the inner edge of the eyelids, but we also measure that the
more rapidly it decelerates, the denser the molecular gas becomes,” said
Kaufman. “This direct measurement of compression shows how the
encounter between the two galaxies drives gas to pile up, spawn new star
clusters and form these dazzling eyelid features.”

Computer
models predict that such eyelid-like features could evolve if galaxies
interacted in a very specific manner. "This evidence for a strong shock
in the eyelids is terrific. It's all very well to have a theory and
simulations suggesting it should be true, but real observational
evidence is great," said Curtis Struck, a professor of astrophysics at
Iowa State University in Ames and co-author on the paper.

“ALMA
showed us that the velocities of the molecular gas in the eyelids are on
the right track with the predictions we get from computer models,” said
Kaufman. “This critical test of encounter simulations was not possible
before.”

Astronomers believe that such collisions between
galaxies were common in the early universe when galaxies were closer
together. At that time, however, galactic disks were generally clumpy
and irregular, so other processes likely overwhelmed the formation of
similar eyelid features.

The authors continue to study this
galaxy pair and currently are comparing the properties (e.g., locations,
ages, and masses) of the star clusters previously observed with NASA’s
Hubble Space Telescope with the properties of the molecular clouds
observed with ALMA. They hope to better understand the differences
between molecular clouds and star clusters in the eyelids and those
elsewhere in the galaxy pair.

The National Radio Astronomy
Observatory is a facility of the National Science Foundation, operated
under cooperative agreement by Associated Universities, Inc.

# # #

This
research is presented in a paper titled “Ocular Shock Front in the
Colliding Galaxy IC 2163” by M. Kaufman et al., published in Astrophysical Journal. [Preprint: https://arxiv.org/abs/1608.02130].

The
Atacama Large Millimeter/submillimeter Array (ALMA), an international
astronomy facility, is a partnership of ESO, the U.S. National Science
Foundation (NSF) and the National Institutes of Natural Sciences (NINS)
of Japan in cooperation with the Republic of Chile. ALMA is funded by
ESO on behalf of its Member States, by NSF in cooperation with the
National Research Council of Canada (NRC) and the National Science
Council of Taiwan (NSC) and by NINS in cooperation with the Academia
Sinica (AS) in Taiwan and the Korea Astronomy and Space Science
Institute (KASI).

ALMA construction and operations are led by ESO
on behalf of its Member States; by the National Radio Astronomy
Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on
behalf of North America; and by the National Astronomical Observatory of
Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO)
provides the unified leadership and management of the construction,
commissioning and operation of ALMA.